River Phoenix is an odd sort of actor who, for no apparent reason -- perhaps in keeping with his strange name -- likes to give interviews upside down. After requesting a drink from his mother (''Yo, Mama-jama, can we have some OJ, pleeze!''), the 5 ft.8 in. 135-lb. beanstalk is hanging by his knees from an exercise bar in his parents' rented home outside San Diego. He is delivering himself of observations he's formed in his long 16 years. Has he done drugs? ''I've tried marijuana a few times but I don't like it,'' River admits as his size-9 feet -- stuffed into a pair of smelly basketball shoes -- kick wildly in the air. ''I really get boring on marijuana. It makes me dull.''
It's probably the only thing that does. As one of the stars of Rob Reiner's Stand By Me, River is being called one of the most exciting young actors on the screen. In what the Washington Post described as a performance that gives the movie its ''center of gravity,'' River plays Chris Chambers, a scruffy, sensitive, cigarette-smoking kid burdened with a family of losers. ''He's trapped in his reputation -- that no- good Chambers kid,'' says Phoenix, whose quiet intensity has been compared to that of Montgomery Clift and Steve McQueen. ''He's a smart kid. He wants so bad to climb above it all. I felt sorry for him.''
The emotional crescendo in Stand By Me comes when Chris, accused of theft, finally confronts the town's contempt for his blue-collar family. ''For my money it's the strongest thing in the film,'' says director Reiner. ''I've seen the movie a thousand times, and every time I see that scene I cry.''
Maybe what makes River so evocative is that he lacks the affectations of the born-to-be-a-star actor. An independent spirit, Phoenix does not have moussed hair or leather-patched blazers, and he makes no precocious displays of industry sophistication. (He claims he doesn't even know who Vanna White is.)
That ingenuousness owes something to his unorthodox upbringing. River and his siblings -- brother Leaf, 12, and sisters Rainbow, 13, Liberty, 10, and Summer, 8 -- are being raised in '60s style by John and Arlyn Phoenix. After River was born in a log cabin in Madras, Ore., where his parents had jobs picking apples, the family migrated across the country in a VW bus. Joining a religious organization, the Children of God, in 1973 the Phoenixes traveled to Mexico, Puerto Rico and Venezuela.
It was there that River and Rainbow formed a singing duet to help make ends meet. Sometimes they performed in talent shows and sometimes, says River, ''at airports and in front of hotels -- ya know, to eat.''
Although John had been named Archbishop of Venezuela by the Children of God, he and Arlyn became disillusioned with the organization and left in 1977. The family made its way back from Caracas, first settling in Florida, then in California.
While John worked as a landscaper and furniture restorer, Arlyn toiled as a secretary. By the time the family reached California, River was beginning to evince some acting ability. Roles in commercials gradually led to TV movies, a TV series, 1982-83's Seven Brides for Seven Brothers, and a feature film, 1985's Explorers.
''We were flower children,'' says John, 39, who with Arlyn, 40, now manages River's career. ''We were full of faith and we loved everybody. I think the kids have learned that. I don't think there is any prejudice in their bones.''
Rob Reiner agrees. ''There's something inside River that his parents are largely responsible for. He's got tremendous warmth; he's obviously been loved quite a bit. His parents have somehow managed to maintain what was pure and good about the '60s morality and make it work. When kids are talented and become successful, you never know if they're going to make the right judgments. Knowing his family, I give River a good shot.''
River gets his next onscreen exposure in December, when he co-stars with Harrison Ford in The Mosquito Coast. Filming the adaptation of Paul Theroux's novel gave River the chance to start a relationship with cast member Martha Plimpton, 15, his girlfriend for the past seven months. They'd met a year earlier, says River, ''but we couldn't stand each other.'' Once filming started, however, ''we realized we both had changed a lot,'' although River can't quite explain how. ''I don't know, we're just cooler, I guess. We both grew a couple of inches.''
River is also at a loss to say how his career will develop, though his imagination and energy should take him far. ''I was in the birth canal for three days,'' he proclaims with a poker face, ''so when I came out, I really shot out.'' If he catapults into the future the way he came out of the womb, River Phoenix is going to be a name to remember.